Mesophase Pitch-Derived Hard Carbon via Intramolecular Oxidation Strategy for Low-Cost Sodium-Ion Batteries

Mesophase pitch (MP) derived from heavy oil is a suitable precursor material for carbon electrodes due to its ash-free nature, low cost, and high carbon yield. However, its molecular structure mainly consists of rich polycyclic aromatic hydrocarbons (PAHs), which are easy to graphitize during carbonization, so as to be not conducive to sodium storage. Herein, an intramolecular oxidation strategy is proposed to realize the disturbance of MP molecules and the introduction of effective oxygen functional groups (C(O)-O) simultaneously, inhibiting the melting rearrangement of MP molecules during pyrolysis. Compared with MP-based hard carbon without pretreatment (reversible capacity of 117.8 mA h g^-1), the optimized MP-based hard carbon exhibits a remarkable specific capacity of 267.1 mA h g^-1 at 30 mA g^-1, with the low-voltage plateau capacity ratio increased to 2.2 times, and excellent cycle stability with a capacity retention of 89% over 100 cycles. Furthermore, the sodium-ion storage mechanism of optimized MP-based hard carbon at different sodium storage stages are identified through in situ Raman. This work not only provides a simple and efficient method for the first time to transform MP from fusion to thermoset but also offers another possibility for low-cost sodium storage.